Which of the following is NOT a commonly used fire extinguishing agent?

Water is indeed a commonly used fire extinguishing agent, particularly effective against certain classes of fires, like those involving ordinary combustibles such as wood, paper, and cloth. However, it is not as universally applicable for all fire types, especially for fires involving flammable liquids or electrical equipment, where using water can cause harm or further spread the fire.

On the other hand, Halon 1301 is a well-known agent effective in extinguishing fires by interrupting the chemical reaction of combustion, making it suitable for use in aircraft and various military applications. DB and CB designations typically refer to specialty extinguishing agents developed for specific fire suppression needs, and they typically aren't used as broadly recognized or common extinguishing agents like water or Halon.

Thus, the correct identification of water as a commonly used agent indicates a misunderstanding of its application across different fire types. However, its limitations in certain circumstances set it apart from more specialized agents, which may make those alternatives more preferred in specific scenarios.